[nicoburns]

Type systems just used to be bad. Anything that forces you to use a class hierarchy to represent an "OR" type (sum types) is painful to work with. Modern languages like TypeScript / Rust / Swift / Kotlin that have sum types are dramatically much nicer.

[giraffe_lady]

Algorithm W is 40 years old and HM has vastly better ergonomics than TS's types.

Why certain approaches didn't catch on until recently, or ever, is an interesting thing to think about but "we didn't know how" is not the story here.

[cubefox]

The "or" (and "and") types in TypeScript are set theoretic rather than algebraic, so they don't require wrapping and unwrapping. It seems to me that they are the ones with better ergonomics.

[giraffe_lady]

It sounds neat when you phrase it that way but my decade of professional experience in typescript has not shown it to be the case in practice. ymmv I guess.

[dboreham]

C also has sum types.

[keyle]

No if you're referring to unions and an enum, it isn't sum types.